Many construction and earthmoving machines use a hydraulic or pneumatic cylinder for moving a work tool such as a bucket of a wheel loader or excavator. The cylinder includes a rod coupled to the work tool, at one end, and a chamber at the other. Hydraulic fluid is provided in the chamber to extend the rod along a linear path, typically extending several meters. The fluid can also be removed from the chamber to retract the rod along the same path. Often the rod location or position along the linear path must be known so that movement of the work tool can be controlled.
Barcodes have been marked on cylinder rods in order to locate the position of the rod. In particular, the rod may be marked with a barcode including non-repeating segments of code, each of which correspond to a different location of the rod. In operation, a sensor is provided in the cylinder adjacent the barcode to identify a particular segment, which is then associated with a corresponding rod location.
In order to locate a rod position with sufficient resolution, a relatively large number of code permutations are often provided over a substantial length of the rod. Such complex markings, however, typically require a relatively large amount of processing power to decode. Although processing power may be reduced with a less complex code, such simpler codes have a limited number of permutations and thus can only be used to determine rod location over a limited range of distances. Otherwise, the code repeats itself and would yield multiple locations for the same code segment.
U.S. Pat. No. 6,556,946 issued to Sewell on Apr. 29, 2003, describes a barcode utilizing a trinary code including a succession of data fields, each of which includes so-called “trits” or characters. Data is encoded by varying the relative amounts of black and white in each character. The barcode described in the Sewell patent facilitates accurate location measurements over a range of rod positions.
In Sewell, the barcode is exposed to either ambient light or light from an optical source. Light reflected off the barcode is detected by a sensor. Data present in the sensor output is read and the rod location is determined based on the data. In order to accurately sense the barcode described in Sewell, however, signals generated by the sensor should be relatively free of noise. Various non-idealities in the optics and electronics for sensing the barcode, however, as well as imperfections in the rod itself (e.g., spurious reflections on the rod surface and excess hydraulic fluid) can introduce noise into the sensor output signal. Often, the amount of noise can be substantial, and the rod location difficult to determine. Accordingly a more robust barcode, which is less susceptible to noise, is required to accurately identify rod positions.